Interpretive Summary: Free-flying insects need stationary visual cues in order to steer their course along an odor plume, e.g., a mosquito flying to feed on its host, or a male moth follows a sex pheromone plume to its emitter/mate. Also, while plume following, insects can only actively control their speed, turning rates and their steering upwind as they fly toward an odor source. Insects fly upwind faster when they fly higher above the ground. The technology is now available to record and analyze these flights to determine if they altered their steering, turning or speed. Here we analyzed video records of moths flying at three heights over floor patterns of transverse stripes and random patterns of equal-sized dots. We found small changes in a number of factors, but the only significant change the moths made was the increased ground speed at greater heights. This resulted in significantly higher airspeed and net speed but these are trigonometric relationships that follow from the increased ground speed since they did not change the course they steered upwind. In these experiments we also altered the number of visual cues (stripes or dots) that the insects could see. This had little to no effect on their flight speed steering or turning, the insects’ natural environment is typically much richer than a laboratory wind tunnel offering insect ample cues to follow an odor plume and locate their host, mate – or pheromone monitoring trap. These findings elucidate one more flight algorithm used by insects during odor-source location, which they do extremely well. Elucidation of these algorithms and understanding their interactions may allow us to disrupt them so insects can no longer locate their mates or hosts. The development of autonomous flight control and odor source location (e.g., gas leaks, explosives) by man-made machines also benefits from elucidation of these algorithms.

Technical Abstract:
Male Grapholita molesta (Busck) were allowed to fly upwind along horizontal sex pheromone plumes in laboratory flight tunnels. Flying males experienced tunnel-width stripes perpendicular to the wind line, or pseudo randomly distributed dots (5cm diameter, equal to stripe width), and their flights were video recorded as they flew at three heights over these patterns at 3 densities (12.5%, 25%, and 50% pattern coverage on a white floor). Moths net speed was faster toward a pheromone source as they flew higher above striped and dotted floor patterns as we have documented previously; however, this is the first analysis of moths’ flight responses to increases in elevation above the floor. We measured the moths’ ground speeds and track angles and then, using the triangle of velocities method, we calculated their airspeeds and course angles and we assessed their crosswind turning frequencies and their cross-tunnel excursion distances between these turns. The moths significantly (P< 0.05) increased their ground speed over floor patterns of transverse stripes and pseudo randomly placed dots. There were trends toward more upwind steered flight (smaller course angles) at the upper height but there were no significant differences (P >0.05) among flight heights. Track angles (flight path angle off the windline), on the other hand, decreased significantly (P < 0.05) when moths flew 40cm above the floor patterns vs. flight at 10cm up. This appears to be entirely due to significantly (P <0.05) lower wind-induced drift when the moths flew faster. Decreasing the density of visual cues by halving the number of dots or stripes and then halving them again had no discernable affect on the moths’ flight parameters at any height, save for the increased interturn distances (P <0.05) above the lowest density (12.5%) dot pattern at the 40 cm flight elevation compared to the 25% dot pattern density, and moths flying over striped patterns at 50% floor coverage had a higher net uptunnel speed at 40cm up than males at the same height flying over 12.5% striped floor coverage. This increased net speed appears to be the result of trends of course angles steered more upwind than over the lower striped-pattern densities along with trends of higher ground speed and fewer turns across the windline. It remains for us to decipher the insects’ ability to use object size, shape and orientation during their upwind flight.